This invention relates to freestanding containers, and more particularly to a freestanding carbonated beverage bottle having a footed base which provides an improved balance of properties in regard to creep resistance, stress crack resistance, impact strength, weight, standing stability and formability.
Over the last twenty years, the container industry for carbonated soft drinks has converted almost in its entirety from glass bottles to lightweight plastic bottles. The evolution of these plastic bottles during that time period has been significant, and a review thereof highlights the critical balance of properties required for producing a commercially successful bottle today.
The 1960's initiated an era of diversification for metal and glass container suppliers into the relatively new, but promising flexible and semi-rigid plastic container market. Through development and/or acquisition, companies like Continental Can Company, Owens Illinois and Sewell developed extrusion blow molding operations to produce high density polyethylene, polypropylene and polyvinyl chloride containers for the growing consumer food and household chemical markets.
At this time enormous growth was occurring in the carbonated soft drink (CSD) industry and was being met exclusively by glass (in larger container sizes) and metal (in smaller container sizes) suppliers because the commercially-available polymers of the period did not offer the critical balance of properties required for carbonated beverages. As such, chemical companies, equipment suppliers and container manufacturers initiated plastic CSD development programs in the late 1960's and identified the following basic criteria as necessary elements in large (i.e., 1, 2 and 3 liter) plastic containers for the soft drink market:
glasslike clarity PA1 adequate CO.sub.2 barrier retention PA1 resistance to volume expansion (i.e., creep) under pressure PA1 no adverse influence on product taste and/or additive migration into the soft drink PA1 significantly improved impact shatter resistance vs. glass PA1 overall economics to permit delivered selling prices equal to or preferably lower than glass. PA1 1) In the early 1980's, the initial 70 gram preform was redesigned to optimize orientation levels and hoop/axial orientation balance. These improvements permitted lightweighting without a loss of bottle creep/stress crack performance utilizing the initial 1976 PETalite base design. PA1 2) During this same time period, efforts to enhance container production rates and maximize graphic space (i.e., label size) on PETalite containers resulted in the commercialization of the improved containers described in Continental's U.S. Pat. Nos. 4,249,667, 4,267,144 and 4,335,821. The '667 patent modified the base hemisphere design to decrease creep by adding straight line sections, producing a reduced base height which also maximized the label panel height (important for marketing purposes). The '144 and '821 patents reduced the mold cooling time by geometrical modifications to the central dome area, above the plane of the feet. All of these enhancements were successfully commercialized without increasing base creep and/or reducing environmental stress crack (ESC) resistance. PA1 3) The advent of rotary re-heat stretch blow molding machines in the mid-1980's (via Krupp of Germany and Sidel of France) led to dramatic increases in production rates and consistency of material distribution in the bottle sidewall. The latter permitted a weight reduction to 58 grams with the same basic PETalite base design introduced in 1976. PA1 ease of formability (processability) PA1 line handling stability (empty and filled) PA1 low stress generation and balanced stress distribution (i.e., minimal creep and no high stress concentration points when pressurized) PA1 efficient use of materials (i.e., lightweight) PA1 no adverse impact on productivity (i.e., minimum mold cooling requirements).
Two polymer material candidates were developed in the early 1970's. Monsanto focused on polyacrylonitrile/styrene copolymer (ANS) containers produced via a two-stage parison extrusion blow and subsequent reheat stretch blow mold process. DuPont focused on polyethylene terephthalate (PET) containers produced via a two-stage preform injection molding and subsequent reheat stretch blow mold process.
Monsanto's ANS bottle made by an extrusion blow process and having an integral champagne base was first commercially marketed (by Coca-Cola in a 32 oz. size) in 1974. Although adequate for clarity, barrier and creep resistance, the bottle exhibited poor drop impact performance, poor economics vs. glass, and was subsequently banned by the U.S. Food and Drug Administration (FDA) in 1976 after migration studies showed the presence of residual acrylonitrile monomer in the beverage after relatively short storage periods. Although controversial, the ban effectively eliminated ANS as a competitor and left PET as the only viable beverage bottle material.
DuPont created polyethylene terephthlate (PET) as a synthetic substitute for silk fiber during World War II. Initial commercial applications were as fibers and flexible films. The polymer was subsequently FDA approved in 1952. PET's clarity, sparkling cleanliness, low cost and excellent strain hardening, orientation and crystallization characteristics expanded its market penetration throughout the 1960's into medical and photographic film, thermoformed semi-rigid wide mouth packages, and other products. In the late 1960's a DuPont chemist, J. Wyeth, brother of Andrew Wyeth the painter, conceived the two-stage preform injection molding and subsequent reheat stretch blow process resulting in the now famous Wyeth U.S. Pat. No. 3,718,229 of 1973. DuPont enlisted Cincinnati Millicron, a machine supplier, in a joint venture to develop and commercialize the new process.
In parallel to these resin developments, Continental Can Company ("Continental") focused on the establishment of low cost conversion systems and container designs. Continental early on targeted a freestanding single material design as a critical element in a low-cost plastic CSD container. It was projected that over time an optimized one-piece design would produce containers faster and with a lower total resin cost and at a reduced overall capital investment vs. two-piece designs (i.e., those utilizing a bottom supporting member or "base cup" of a separate molded polymer). The Adomaitis patent (U.S. Pat. No. 3,598,270) granted to Continental in 1971 disclosed the world's first plastic free standing looted pressurized plastic container, now known as the "PETalite" container.
In the 1970's, Continental focused on a two-liter container design, anticipating correctly the CSD industry's desire to upsize "family" packages beyond that safely achieveable with glass (one-liter maximum). In 1976, Continental commercialized the first six-foot PETalite (one-piece) two-liter PET bottles for Coke and Pepsi. All remaining PET suppliers (Owens Illinois, Sewell, and Hoover Universal (now JCI), etc.) chose to develop two-piece (bottle and base cup) containers.
The new PET beverage bottles, both one and two-piece, were an immediate commercial success as consumers favored the light weight, large size, shatterproof safety and convenience over competitive glass bottles. By 1982, virtually all of the glass CSD packages above 16 ounces had been displaced by PET.
The 1980's saw significant increases in productivity and reductions in container weight and selling price for all sizes, both one and two-piece constructions. Several key technical improvements were commercialized by Continental to improve the viability of one-piece CSD containers in the marketplace, including:
Further lightweighting attempts below 58 grams were halted when test market containers exhibited unacceptable levels of environmental stress crack (ESC) initiation and occasional propogation through the bottle sidewall (i.e., yielding unacceptable field leakers). ESC generation is a relatively complex phenomenon that occurs when low orientation regions of a PET container are exposed to high levels of stress (due to internal pressurization) in the presence of stress crack initiation agents, such as line lubricants (utilized on the filling line), moisture, corrugate, shelf cleaning agents (utilized by grocery stores), etc. Highly biaxially oriented PET, such as that in the bottle sidewall regions, is extremely resistant to ESC formation. However, the lack of stretch induced crystallization in the low orientation, highly stressed regions of a freestanding base can initiate chemical attack on the exterior surface (which is in tension when pressurized), micro-cracking, and under severe conditions, crack propogation through the container wall.
To address this ESC concern, Continental undertook a development program to redesign/improve the original PETalite base to permit further lightweighting. Several critical elements to the overall commercial success of a freestanding base were considered:
After significant development efforts, a five-foot base design was achieved, as described in Krishnakumar U.S. Pat. No. 4,785,949, which issued in 1988. The five-foot retained the basic foot design of the original PETalite base, but with a significant increase in the rib area defined by the hemispherical bottom wall, and further allowed a 4 gram weight reduction. A 54 gram, two-liter five-foot bottle was commercialized having improved field performance in substantially all respects over the original six-foot PETalite (Adomaitis '270) base design.
In the late 1980's, other competitors, recognizing the cost disadvantages of the two-piece design and the significant recycling advantages of the PETalite approach, initiated one-piece development efforts of their own. A freestanding PET bottle patent was issued to Owens Illinois as Chang U.S. Pat. No. 4,294,366. The Chang patent describes a generally elliptical (rather than a generally hemispherical) transverse cross section through the rib area. The hemispherical approach, however, is preferred as it provides improved geometrical resistance to deformation under pressure (i.e., creep) vs. an ellipse. Owens Illinois ultimately exited the CSD PET market and as such, the Chang '366 base was never successfully commercialized.
Powers U.S. Pat. No. 4,867,323 issued in 1989 to Hoover Universal (now JCI) and focused primarily on maximizing the foot pad width and diameter for improved line handling. However, narrow U-shaped ribs provided high stress concentration areas and susceptibility to stress cracking. The low rib cross-sectional area yielded poor resistance to bottom deformation under pressure, yielding excessive height growth and product fill point drop (i.e., the appearance of low fills on the store shelves). The '323 container was never successfully commercialized.
Behm U.S. Pat. No. 4,865,206 issued in 1989 again to Hoover (now JCI), and attempted to improve on the '323 patent by increasing the number of ribs from three to five, thus increasing the rib area and reducing the pressure deformation (creep), albeit to a limited degree. Again, however, foot size is stressed over rib width and base creep remains a problem. In fact, to accommodate the creep problem an angled design is provided for the foot pads which move downward under pressure into the foot "plane" as the base itself deforms outwardly. The deep, wide foot pads themselves are difficult to form and most commercial bottles show evidence of underformation (potential rockers) and/or stress whitening (visual defect due to overstretching/cold stretching). Although marketed in the U.S.A., the relatively heavy 56.5 gram two-liter container is found only in the cooler latitudes where ESC problems are less of a concern (lower temperatures produce lower stress levels and reduce ESC propogation).
Walker U.S. Pat. No. 4,978,015 issued in 1990 to North American Container, and once again focused primarily on line handling stability by maximizing the foot pad contact area. Base creep and ESC resistance are severaly compromised by the narrow, sharply radiused "U-shaped" inverted ribs. When commercialized this design would be expected to exhibit poor formability and inferior thermal performance in warm climates.
There have been numerous other proposed designs for freestanding carbonated beverage containers, e.g., U.S. Pat. Nos. 3,727,783 (Carmichael), 5,024,340 (Alberghini), 5,024,339 (Riemer) and 5,139,162 (Young et al.), but none of these has achieved an improved combination of properties nor been the commercial success of the Krishnakumar five-foot design.
Despite the success of the Krishnakumar five-foot design, Continental has continued developmental activities to further optimize freestanding PETalite container technology. These efforts have produced the new container base design of this invention.